The present invention relates to a display technique that contributes to exerting anti-counterfeiting effects.
Generally, valuable stock certificates, such as gift tickets or checks, cards, such as credit cards, cash cards or ID cards, and certificates, such as passports or driver's licenses, are each adhered with a display having visual effects different from those of a normal printed object, for the purpose of preventing counterfeiting of these articles. Further, recently, circulation of counterfeit articles besides these articles is also becoming a social problem. Therefore, there are increasing opportunities of applying similar anti-counterfeiting technique to such articles as well.
As described in the specification of U.S. Pat. No. 5,058,992, a display exerting visual effects different from those of a normal printed object uses a method of arranging a plurality of relief-type diffraction gratings having grooves whose longitudinal directions or grating constants (i.e. pitches of grooves) are different from each other, and displaying an iridescently changing image. However, lots of articles needing measures for preventing counterfeit have used such a display that includes relief-type diffraction gratings. As a result, this technique is now widely known. Along with this, there is a tendency that counterfeit articles that have incorporated this technique are increasingly common. Therefore, it is now more difficult to achieve sufficient anti-counterfeiting effects using a display only characterized by iridescent light that is due to diffracted light.
In order to achieve more sufficient anti-counterfeiting effects, JP-B-4983899 discloses a display that realizes special visual effects which are different from those of conventional relief-type diffraction grating. JP-B-4983899 discloses a display provided with a concavo-convex structure which is configured by arranging a plurality of convexities having an upper surface that is substantially parallel to a base surface or a plurality of concavities having a bottom surface that is substantially parallel to the base surface, and a smooth portion that is substantially parallel to the base surface. This display has a function of displaying a mixed color which is configured by light of a plurality of wavelengths, in a predetermined direction. The image shown by the display hardly exhibits color change in an iridescent manner in accord with the change of position of an illumination source or the position of an observer, but realizes visual effects that are different from those of the conventional relief-type diffraction grating aimed to anti-counterfeit. As a result, a display exerting high eye-catching effects (effects of attracting people's attention) and high anti-counterfeiting effects can be realized.
However, the display disclosed in JP-B-4983899 displays only a mixed color that is configured by light of a plurality of wavelengths which are obtained from the concavo-convex structure configured by convexities or concavities substantially parallel to the base surface and a smooth portion substantially parallel to the base surface. Therefore, when the display is observed from the back side as well, a color that is the same as the one observed from the front side of the display is observed. The configuration shown in JP-B-4983899 is not able to achieve the visual effects that are different on the front and back and desirable for obtaining higher anti-counterfeiting effects.